Technical Field
The present invention relates to a turbo-charging system of an engine.
In particular the present invention relates to a turbo-charging system of an engine, of the type connected to the intake phase of the engine.
Description of the Related Art
It's well known that internal combustion engines having pistons can be distinguished in:                compression ignition diesel engine, characterized by the introduction of the finely atomized fuel at the end of the compression and therefore at its power on, which occurs spontaneously; and        gasoline carburetion engine with carburetor or injection and having spark ignition and subsequent flame propagation.        
In the gasoline carburetor engine the mixing of the fuel and the combustive is usually made through a carburetor externally to the engine, causing the continuous injection of fuel into the intake pipe in which the air spirated from the pistons flows. In the gasoline injection engine, compared to carburetion through a carburetor, there is the direct injection of fuel into the cylinder during the intake phase and in part during the compression, that's advantageous in terms of economy of consumption.
In addition, the direct or indirect injection has several advantages compared to carburetion by a carburetor:
1) increasing the volumetric efficiency, for the increased size of the supply pipes;
2) elimination of backfiring danger, because the pipes are fuel free;
3) improvement of dead space washing, which is carried out with air and can therefore be prolonged without wasting fuel during exhaustion;
4) fuel economy because a good combustion is possible;
5) more vivid acceleration, because the fuel does not have to travel through long pipes; and
6) greater ease of design of the engine, since the position of the injection system (pump, any distributor, regulator, injectors) is not strictly bound to that of the motor.
The majority of the cars are equipped with four-stroke engines. An operation cycle, shown in FIG. 1, consists of the following four phases:                The intake phase, in which, when the piston is lowered from the top deadlock to bottom deadlock, and draws in the cylinder, through the appropriate intake valve open, an air current which, passing through the carburetor, is enriched of fuel (section AB—isovolumic transformation);        the compression phase, in which the piston, moving from the bottom deadlock to the top deadlock, compresses the air/fuel mixture in the cylinder, the valves being closed (segment BC-transformation ideally adiabatic isentropic);        the expansion phase, which represents the useful work done by the engine, in which the expansion produced by the combustion of the air/fuel mixture, pushes the piston from top deadlock to bottom deadlock;        the combustion phase, in which shortly before the piston has reached the top deadlock the spark between the electrodes occurs by kindling the mixture that surrounds them, so the combustion propagates, with rapid increase of pressure, to the rest of the mixture (section C);        the expansion phase, in which the piston descends, allowing the expansion of the combustion gases (section DE-adiabatic transformation);        the exhaustion phase, in which before the piston has reached the bottom deadlock, the exhaust valve opens, allowing the gases that are still in the cylinder, at a pressure greater than the external resulting from the residual flame propagation, to discharge but still keeping the cylinder full of flue gas at a pressure near atmospheric pressure (section EF); and        the expulsion phase, in which the piston, moving from the bottom deadlock to the top deadlock, pushes out, through the appropriate exhaust valve, the exhaust gas, only the gas filling the dead space remaining in the cylinder (section FA).        
The power delivered by the engine may be increased in several known ways.
For example, the expansion of the cylinder capacity allows a power output increase, because more air is available in a larger combustion chamber as more fuel/air mixture can be burnt. This expansion can be achieved by increasing the number of cylinders or the volume of each individual cylinder. In general, this results in larger and heavy engines.
Another possibility to increase the power output of the engine is to increase its speed. This is possible by increasing the number of ignition strokes per unit of time. Due to the mechanical limits, however, this type of power increase is sparsely used. Furthermore, the increase of speed does grow exponentially friction and inertia, resulting in the decrease of the engine efficiency.
Another possibility is to supercharge the engine. As is known, in the mechanical motoring, different solutions are adopted for supercharging an engine, including the use of a turbocharger.
A turbocharger coupled to a vehicle engine consists of a turbine and a compressor connected by a common shaft supported on a bearing system. The turbocharger converts the energy lost by the exhaust gas into compressed air pushed into the engine and this allows the engine to produce more power and torque.
In engines without turbochargers, the engine operates as a naturally aspirated engine. In fact, the combustion air is drawn into the cylinder during the intake phase and is sucked from the external environment, which has a pressure equal to the environmental pressure. However, the combustion air is conditioned by the altitude as the oxygen decreases with increasing altitude. Conversely, in supercharged engines, the combustion air is compressed. Consequently, at fixed altitude, a greater amount of air and, therefore, oxygen can enter the combustion chamber. This involves the combustion of a greater quantity of combustion air, and the increase of the engine output power proportionally to the cylinder capacity.
Basically, we can distinguish the mechanically supercharged engines from turbocharged engines with exhaust gas at fixed or variable geometry.
In the first case, the combustion air is compressed by a compressor driven directly from the engine, by means of mechanical organs, such as belts, gear trains, etc. However, the growth of the output power is partially attenuated by the dispersion caused by the compressor operation. The power required for operating a mechanical compressor needs a part of the power delivered by the engine.
In the fixed geometry turbocharger the energy of the exhaust gas, which would normally be lost, is in part used to drive a turbine. In fact, there is a compressor mounted on the same axis of the turbine, that compresses the combustion air and then supplies it to the engine, without any mechanical connection to the engine, as already described. The body of the turbine comprises two components: the “blades wheel” of the turbine and the “enclosure/housing”. The exhaust gas is guided into the turbine wheel from the housing and the energy present in the exhaust gas let the turbine rotate. Once the gas has passed through the blades of the turbine wheel, it comes out from the exhaust exit. The revolutions of the turbine wheel are determined by the speed of the engine, so if the engine is at a minimum mode the wheel rotates at a minimum speed. Due to the pressure on the accelerator, the wheel starts to spin faster due to the passage of a greater amount of air through the housing of the turbine.
Also the system of the compressor is constituted by a housing body and an impeller.
The impeller of the compressor, or “wheel” of the compressor, is connected to the turbine by a forged steel axis.
The combustion air is guided into the compressor wheel from the housing. Once compressed, the air leaves the body from the output of the compressor and flows into the engine cylinders. At its entry into the compressor, the air has a temperature equal to the atmospheric one but, due to a phenomenon of thermal transmission, it comes out at a temperature higher than 200° C. The temperature increase is determined by the contact of the combustion air with the body of the turbine, crossed by the exhaust gas at high temperature that drive the turbine, and, for the same volume, the temperature increase causes the decrease in the amount of oxygen present in the air, thus lowering the stoichiometric ratio. The temperature increase of the air can be counteracted by cooling the same, downstream of its exit from the compressor, by means of a heat exchanger called “intercooler”.
A fixed geometry turbocharger comes into operation when the force provided by the exhaust gas is such as to let the turbine rotates, depending on the number of engine revolutions. Therefore, the fixed geometry turbochargers are good to use for the low, medium, or high engine revolutions.
A more effective but more complex method of turbocharging is the variable geometry method, which consists in the use of a turbine, which has the ability to capture, thanks to a system of movable blades, all the exhaust gas from a minimum condition of engine operating at low speed to a maximum condition of high speed.
However, the turbochargers currently used have various problems such as weight or size; require the installation of exhaust manifolds for each specific engine for connecting the same to the turbocharger; determine problems of negative environmental impact resulting from the failure emptying of the combustion chamber after a combustion cycle and the consequent accumulation of combustion residues in the subsequent cycles, caused by the need to restrict the cross section of the exhaust manifold in order to increase the speed for starting the turbine. Further, since the high temperature of the exhaust gas reaches 800/1000° C., it is required the use of blades of the turbine made with materials resistant to high temperatures, such as cast iron. However, this leads to an increase of weight of the turbocharger and makes necessary the use of lubricating oils, specific for turbo engines, resistant to high temperatures, which are pollutants. Moreover, relatively to the volumetric compressor a problem is constituted by the absorption of power of the engine caused by the need of the engine to drag all its the mechanical components.
A first solution to these problems has been described in the patent FR 2610672, which refers to a system interposed between the carburetor and the engine, consists of two turbines of different diameters connected on the same axis and rotating at the same speed and operating on an air-fuel mixture.
Although advantageous and able to supercharge an engine, the above described solution does not include a turbine-compressor system.
Although advantageous under many aspects, these solutions are not able to send the fluid to the impeller in a certain direction and cannot collect the fluid discharged from the first impeller conveying it in a suitable direction to the second impeller. Indeed, such a solution does include neither means which can send the fluid to the impeller in a given direction nor means collecting and guiding the fluid from the first to the second impeller, then most of the energy of the air is dispersed. Moreover, once crossed the first turbine at the ignition, the air goes partly through the second turbine and partly bumps on the turbine blades interfering in the rotation of the device, which can not exert work on the air. In addition, in the internal combustion engine, a pulsing pressure is determined during the alternating operation of the cylinders, at the time of the air intake in the intake high volume manifold. For these reasons and for the fact that the device described above is positioned very close to the valves, this device has not a good efficiency. Furthermore, a certain overpressure can be produced with such a device, due to the fact that the increase of air is dependent only on the ratio between the diameters of the two turbines. Furthermore, for the functionality, the positioning of the device between the carburetor and the engine, in an engine with carburetor, causes the following disadvantages: increased danger of backfiring, because the system is connected to the carburetor; and greater difficulty of the engine design, because the position of the system is closely linked to the position of the engine.
A second solution is presented in the patent DE 102010043800 published on Jun. 1, 2011, on behalf of Denso Corporation, which describes a turbocharger having a turbine driven by exhaust gases, disposed in the passage of the discharge opening, and a compressor disposed in the inlet passage. There is also a catalyst disposed in the passage of the discharge opening, on one side of the turbine, to clean it from the exhaust gases. This solution refers to a flue gas recirculation system which consists in putting into circulation a small percentage of the exhaust gases by passing them from the exhaust manifold to the intake manifold, in order to reduce part of the pollutants present in the exhaust gases. To obtain this flue gas recirculation, during the terminal phase of the discharge and the initial phase of the aspiration a special solenoid or hydraulic valve (EGR) is used, which is controlled by the engine control unit via a signal, allowing adjustment of the amount of exhaust gas by the collectors. The gases are recirculated into the intake manifolds and sucked into the engine. Together with the exhaust gases, the circuit also takes the gas coming from the carter and due to filtering through the piston rings and the engine evaporation oil.
Although advantageous under many aspects, this solution has several problems. Since it is a turbocharger driven by exhaust gases, its performance is compromised, not only for the variability of the characteristic ratio of the turbine operation, but also for the interference of the various discharges from different cylinders and also from the same cylinder, due to its inertia. Therefore, this method should be used such as the distributors of the turbine are reached separately by discharges of the various cylinders or such that the same sector of the distributor is reached only by the gas of cylinders whose exhaust phases do not overlap.